Medical infusion controllers now provide microprocessor control of intravenous fluid delivery rates. These devices include sensors that monitor fluid flow rate and then adjust that rate accordingly. Despite the automatic nature of these devices, they typically must be manually set by the operator to indicate to the microprocessor whether the particular infusion set in use contains a 10 cc or a 60 cc cannula. Thus, there is a potential for serious human error. One prior art device has a protruding spring-loaded retractable pin switch on the controller. 10-cc and 60-cc cannula drip chamber are distinguished by the presence or absence of an aperture to receive the pin. If the aperture is absent, the pin is urged into the retracted position. If the aperture is present, the pin fits into the aperture, and thus does not retract. The two pin positions are associated with two different modes of operation. It can be seen that there is room for error, inasmuch as misalignment or malfunction of the pin can cause incorrect data to be signalled to the controller, by either retracting even in the presence of an aperture, or by failing to retract even in the absence of an aperture.